Operational amplifiers are widely used in the electronics industry because of their many excellent circuit characteristics including high open loop gain, high input impedance, and low output impedance. Operational transconductance amplifiers, are similar to the operational amplifiers generally, but exhibit high output impedance. General applications of operational amplifiers include circuit configurations such as voltage and current amplifiers, differentiators and integrators, active filters, oscillators, and analog-to-digital and digital-to-analog converters. To realize these different circuit configurations, operational amplifiers are used in conjunction with positive and/or negative feedback in combination with passive and/or active elements.
Operational amplifiers are also widely used to function as voltage comparators, wherein typically, a reference signal is applied to the inverting input, and the voltage to be compared is applied to the non-inverting input. If the magnitude of the voltage to be compared is greater than the magnitude of the reference signal, the output of the comparator equals the positive supply voltage. If the magnitude of the voltage to be compared is less than the magnitude of the reference voltage, the output of the comparator equals the negative or ground supply voltage. An inverted voltage comparator may be provided by simply transposing the signals at the inverting and non-inverting inputs. Using the operational amplifier as a voltage comparator requires no external components or feedback, and its output only has two states of high and low.
The operational amplifier as utilized in the realization of a variety of circuit functions may be manufactured in bipolar or Complementary Metal Oxide Semiconductor (CMOS) technology or some combination thereof. The CMOS implementation is desirable for its low power consumption characteristic. Also, operational amplifiers are increasingly being integrated onto chips that merge digital and analog functions together with an increasing number of devices.
Operational amplifiers are biased and connected to operate with different characteristics, for example Class A, Class B, or Class AB amplifiers; the operations of which are well known to one of ordinary skill in the art. Specifically, for example the Class AB operational amplifier requires bias voltage and current to set the operating point of the amplifier. As a result, additional circuitry has to be incorporated therein to provide the proper bias voltage magnitudes. Still further, the class AB operational amplifiers require large asymmetric differential inputs to generate asymmetrical bias currents flow through the amplifier. This large asymmetric bias current causes an undesirably high common mode current, internal to the operational amplifier circuitry, which might flow depending upon the given circuit application. Unfortunately, the undesirably high common mode current causes a drain of battery power, which for many applications, such as in radio pagers, is to be avoided since battery life is a major design concern.
Thus, what is needed is a class AB transconductance amplifier that does not require additional circuitry for generating large asymmetrical bias conditions which drains battery power because of the high common mode current flowing as a result of the operating point of the transconductance amplifier.